The development of low-loss multimode optical fibers has resulted in interest in the light emitting-diode (LED) as a telecommumications optical source. Fiber-optic cables are being used in applications requiring freedom from electromagnetic interference, low weight, and the ability to tolerate high temperature environments. The attractiveness of the optical transmission of digital data is a function of the bandwidth obtained.
The frequency limitation in driving an LED is theoretically set by the carrier lifetime, .tau..sub.c, but is practically established by the ability to supply charging current to the LED's capacitance. With presently available diodes, carrier lifetime (f=1/(2.tau..sub.c), theoretically allows operation at more than 200Mhz. The data rates for practical circuits, however, have only been about 100Mhz.
Frequency response, however, is dependent on capacitance, which is a function of area, which determines current handling capability. With the selection of a high capacitance LED, any driver would suffer a speed reduction over the low capacitance case. To obtain high speed, the capacitance of the device must be minimized. This is the distinguishing feature of high speed LED's. The capacitance of high frequency LEDs may vary, typically from the 100 pF to the 1000 pF range during the switching time. Currents up to 200 mA are required to charge this capacitance at 100Mhz (1000 pF is about 1.6ohms reactive at 100Mhz).
LEDS can be modulated directly by step-recovery diodes, Gunn-effect diodes or Trapatt diodes at frequencies of 100Mhz and higher. But a forward-biased LED has a non-linear low impedance. One way to achieve maximum speed is to drive the LEDS directly from a low-impedance source (approximately equal to r.sub.d). However, such a brute force method is not ideal for solid state circuits since the efficiency is low.
To prevent the LED rise time from limiting the system bandwidth, the following rule of thumb should be applied: t.sub.r (LED).ltoreq.0.35/(2.2BWreceiver). Usually the bandwidth restriction should be assigned to the photoreceiver to maximize signal to noise and sensitivity.
A large number of circuits have evolved for the purpose of driving LED's. Typical high frequency applications require that currents in the 25-200 mA range be put through the diode. A representative circuit for driving an LED is the emitter-coupled video stage. Its basic function, however, as a fast current switch is well recognized and is exploited in the emitter-coupled-logic (ECL) family. In using the discrete version of this circuit, the designer would select a relatively costly device to obtain high speed in the 50-200 mA range, or would parallel two or more devices to obtain a composite Q.sub.2 stage where the current load would be shared. The base of Q.sub.2 would normally be biased to match the video stage to the logic type used to develop the signal.